Grasp genuine source of investment with the Home Counties (7)
I believe the answer is:
realise
'grasp' is the definition.
(I've seen this before)
'genuine source of investment with the home counties' is the wordplay.
'genuine' becomes 'real' (associated in meaning).
'source of' suggests taking the first letters (the start - or source - of the word).
'with' means one lot of letters go next to another.
'the home counties' becomes 'SE' (the Home Counties are in the South East of England).
The first letter of 'investment' is 'i'.
'real'+'i'+'se'='REALISE'
(Other definitions for realise that I've seen before include "work out" , "See the light" , "Come to know" , "Ears lie (anag.)" , "Become fully aware of" .)